Activation of LA-N-2 cell phospholipase D by amyloid beta protein (25-35)

Amyloid beta protein is the major protein component of neuritic plaques found in the brain of Alzheimer's disease. The activation of phospholipase D by amyloid beta protein (25-35), quisqualate and phorbol 12,13-dibutyrate was investigated in LA-N-2 cells by measuring phosphatidylethanol formation. The activation of phospholipase D by quisqualate and A beta P (25-35) was calcium-independent. The A beta P (25-35) and quisqualate activation of phospholipase D appeared to be mediated through a pertussis toxin-sensitive GTP-binding protein. Phospholipase D activation by A beta P (25-35), quisqualate and phorbol dibutyrate was not blunted by the protein kinase C inhibitors, staurosporine, H-7 and RO-31-8220. However, it was abolished by overnight exposure to phorbol dibutyrate. This activation of phospholipase D was prevented by the tyrosine kinase inhibitor, genistein but not by tyrophostin A. Several excitatory amino acid antagonists were tested for their ability to prevent the phospholipase D activation by quisqualate and A beta P (25-35). Only NBQX was effective with an IC50 of 75 mu M for A beta P (25-35) and quisqualate. Activation of phospholipase D by A beta P or quisqualate was absent in LA-N-2 cells previously desensitized by quisqualate or A beta P (25-35), but the activation by phorbol dibutyrate was unaltered. The responsiveness to A beta P and quisqualate in previously desensitized cells reappeared subsequent to a period of resensitization. The observations with the antagonist NBQX, and the desensitization and resensitization experiments, are consistent with a receptor occupancy mediated activation of phospholipase D by quisqualate and by A beta P (25-35).